The fuel injection that rests on top of your Corvette's engine has gone through several stages of development. Each step of the way has been fueled (pun intended) by a continuing search for more power, fewer emissions, and better driveability. Fuel injection has progressed from the first mechanical fuel-injection (Ram Jet) units, which operated much like a carburetor, to the computer-controlled systems that seem to almost think for themselves. This transformation did not happen overnight, though. There have been several milestones in the procession toward perfect fuel injection.
Follow along and we'll take a look at the milestones that became stepping stones for the next generation.
The mechanical fuel-injection (Ram Jet) systems from Rochester were the first fuel-injection units for Corvette. They were designed to meter fuel according to the intake vacuum readings but, instead of allowing the fuel to be drawn into the intake according to the venturi principle, a regulated amount of fuel was introduced directly above the intake valve under considerable pressure. While still subservient to an engine's vacuum signal and capable of controlling only fuel, for the times, the system was a major improvement over carburetors. They controlled fuel better and were not subject to fuel sloshing in the float bowls during racing situations as was the carburetor.
GM engineers introduced the new Ram Jet fuel injection to the Detroit Section of the Society of Automotive Engineers, and touted it as capable of correcting carburetor deficiencies, improving fuel economy, and reducing hydrocarbon pollution. While it did help fuel delivery, several Achilles' heels hindered the system. Tuning it was one of the major problems. Being mechanical fuel injection, it did not self-adjust to accommodate changing situations. Changes in temperature and altitude could turn a great-running engine into an emissions-belching anchor.
Compared to today's computer-controlled systems, the mechanical fuel-injection systems are relegated to history and occasional drives to and from shows. They have a strong following, and Corvettes originally sporting these systems are highly sought after by collectors. There are subcultures within Corvettedom that deal exclusively with these early Rochester mechanical fuel-injection units. If the owner doesn't mind taking the necessary time to understand and care for these early FI units, driving a car with a Ram Jet can be exhilarating.
And driving them is probably the best thing you can do for them. They are a vented system, which means that if you allow them to sit for extended periods of time, the fuel inside the fuel meter will evaporate, leaving behind a film of varnish. Eventually, this buildup will start to affect the way the system operates, confirming the rumors about why so many of these systems were discarded in the first place. Whether you swear by the performance of these early Fuelie Corvettes or you swear at the mechanical fuel-injection systems, you'll have to agree this was the means by which GM broke the 1hp/ci barrier, and it took a fuel-injection unit to do it.
The next step in the pursuit of fuel control arguably wouldn't be considered EFI, although it did open the door for electronics in fuel control. Beginning in the '80 production year, GM installed computer-controlled carburetors on those 305ci Corvettes bound for California. Earlier GM cars (non-Corvette) had the Computer Controlled Catalytic Converter (C-4) system that controlled only the air/fuel mixture by controlling the main metering rods inside the carburetor to allow better operation and efficiency of the catalytic converters.
The '80 and '81 Computer Command Control system is an expansion of the C-4 system. The CCC system monitors up to 15 engine and/or vehicle operations for controlling up to 9 engine and emissions-control systems. In addition to regulating the air/fuel ratio, the CCC system controls spark timing, AIR management for optimum catalytic converter efficiency, and torque-converter lockup. Obviously, the Computer Command Control system was another step toward today's technology. The programming and computer hardware for the CCC system was nowhere near what is necessary for operation of today's Corvette, and the speed at which the processors operate in today's Corvette wasn't even dreamed about when the CCC system was first introduced. At that point, emissions concerns were the main reason for the evolution and reintroduction of fuel injection into the Corvette. The engineers at GM first had to learn how to keep the feds happy; only then could they devote their precious time to getting more power out of the engine. Lucky for us, performance and emissions are both part of the two-sided fuel-injection coin.
Squirters were back in '82, and this time they were electronically controlled. The tunability issues that plagued the earlier Ram Jets were being addressed by a computer. The Cross-Fire Injection (RPO L83) used two throttle bodies on top of the intake manifold. The computer to drive the system was based on a Cadillac DFI unit and worked with GM's Computer Command Control. CCC had been refined in '82 to the extent that it had the ability to make 80 adjustments per second, compared to the previous year's 10 adjustments per second. These advancements provided a mere 10 hp for Corvette, but it was the beginning of the upswing and it provided a quick means for getting fuel injection back into the Corvette until the Tuned Port Injection was ready for production.
Cross-Fire Injection was carried over for the extended production year of 1984, and another 5 hp entered the engine compartment, topping off the Cross-Fire's engine horsepower rating at 205. History has proven that the Cross-Fire is a reliable design with the proper tuning and maintenance and, with some tweaking to the engine and fuel-delivery system, it can show a stock Tuned Port to the door. Due to its limited production, the Cross-Fire has a certain appeal for those who want more than "just another TPI Corvette." Whether you prefer the shark body style or the late-model (C4) body lines, you can get either with a Cross-Fire, since they were installed in the last sharks ('82) and the first late-models ('84). It may turn out to be the last engine that will ever have the privilege of spanning two generations of production.
Advancements in fuel delivery were coming fast. Within two production years, the Cross-Fire was shelved and replaced with Tuned Port Injection (TPI). The benefits of TPI were almost immediate. Horsepower increased to 230 at 4,000 rpm, and the torque numbers jumped to 330 lb-ft at 3,200 rpm. It was clear that GM was addressing performance and emissions concerns.
Rather than rely on vacuum from the intake like the Ram Jet and Cross-Fire, GM began using MAF sensors on the '85 production models. This gives a better reading of exactly how much air is entering the engine and, in turn, how much fuel should be mixed with it. The '85-'89 TPI Corvettes use a MAF sensor to help regulate fuel, but in '90 they went back to using intake vacuum to regulate the amount of fuel the engine needed. Called "Speed Density," this new system calculated the engine load by sensing Manifold Absolute Pressure. The computer combines the MAP with the Intake Air Temperature and other engine parameters, such as volumetric efficiency and EGR rate, to calculate the mass of the intake air.
Another change that took place with '89 production was the elimination of the Cold Start Circuit that introduced additional fuel into the intake to richen the mixture under cold-starting situations.
The injector styles changed during TPI production. For '85 and '86 production, GM used Bosch injectors in the Corvette. During all of '87 and part of '88, Lucas injectors were installed. In '89, GM went to Multec injectors and used them up through the end of C4 production. GM switched to the Multec injector partly because they were a ball design instead of the pintle like the Bosch-style injector. This means that the ball constantly rotates, which helps eliminate buildup and gives the injector a different seating surface each time, reducing wear on the seats. Also, the Multec injector is higher in the intake, which prevents fouling the tip of the injector. The Multec injector is designed to allow the fuel to flow over the coils of the injector, which helps cool the coils. This can pose a problem if chemicals are used to clean the injectors. Since the fuel flows over the injector coil, so does the injector cleaner. Harsh chemicals in the injector cleaner can deteriorate the protective coating on the coils of the injectors and destroy the injector.
An approved injector cleaner is available from GM; but most of the time, if the injector is not working properly it will need to be replaced. Using good quality fuel is one of the best ways to ensure long life for your fuel injectors. This eliminates the need for cleaners and helps prevent water in the fuel system. When changing injectors, be sure to install the injectors that have the same resistance as the originals. The Multec injectors have around 12.8 ohms of resistance. You can change injectors within a couple of ohms difference, but if you start going too far away from the originals, you'll have problems. Even performance injectors should have around the same ohms of resistance as the originals.
In the pursuit of more performance, GM turned away from the long, tuned tubes of the TPI system and went back to a shorter intake runner. The TPI intake runners were 21 inches long, which provided excellent midrange power; but the engine stopped making power at 4,000 rpm. For the next generation of fuel injection, GM shortened the intake-runner length dramatically to about 3 inches. The LT1 was introduced in the '92 production year. Once again, a base Corvette engine was breaking the 300hp barrier with its short runners and speed density system. The LT1 has a much flatter torque curve than the TPI system, giving up some torque around 3,000 rpm to the TPI, but picking it back up again above 3,500-3,600 rpm. The LT1 engines started production using a MAP sensor for load detection but, after '93, LT1 engines used a MAF sensor with a MAP sensor as a backup system. The '96 LT4 engines also used this MAP/MAF system for fuel mixing.
The implementation of Sequential Fuel Injection was another change for the '94 production year. Instead of all of the injectors on one side of the engine firing at the same time whenever one of the cylinders on that bank required fuel, the SFI fuel system fired the injector for each cylinder independently and only when that cylinder required fuel from the injector. This helped reduce emissions, increased fuel economy, provided less variation between cylinders, produced a smoother idle, and allowed for rpm limitation because of its ability to cut fuel to one cylinder at a time. The implementation of SFI was a great step toward a more controllable fuel-delivery system.
This probably won't be a surprise to most of you, but the ZR-1 didn't play by any of the rules when it came to the exclusive fuel-injection system for the LT5. It always used a MAP sensor, but never used a MAF sensor like the other '94 or '95 Corvette engines. Also, when GM was making the transition between long and short intake runners, the LT5, unbiased, used both. The longer runners were used in everyday driving situations, and the shorter runners were opened when the secondary air inlet control was activated.
Another set of injectors was used and activated by the ECM when the Power Key on the console was turned on to include the proper amount of fuel for the additional air introduced when the secondary air inlets were activated. Also, all ZR-1s ('90-'93 included) used Sequential Fuel Injection instead of Multiport Fuel Injection. This improved the ZR-1's driveability and fuel mileage.
The '97 and '98 Corvettes had return lines with an external fuel-pressure regulator to keep the proper fuel pressure. '99 and later Corvettes use a semi-returnless fuel system. The fuel pressure regulator on a semi-returnless system is located with the fuel filter in the tank. You can see the difference by looking for the fuel lines going through the engine covers on the driver side. If there is one line going through the cover, it's a '99 or later style; if there are two lines (one pressure, the other a return line to the tank), it's the earlier style.
Without carrying over the technology from the ZR-1, GM did the next best thing. It designed an intake with runners that were a compromise from the long Tuned Port and the short LT1 and LT4 engines. The result was the LS1, which was released for '97 Corvette production.
Like the TPI runners, the LS1 intake runners cross over to the cylinder on the opposing bank of the engine, but they are much closer to the LT1 in length at around 10 inches. Also, the port configurations where the intake meets the head is different from previous designs. It is much taller than the ports on any other small-block. It's designed to have good flow while still retaining sufficient velocity at lower rpm, which helps the engine produce more torque. In '01, GM improved this intake design for the LS6 that was installed in Z06 Corvettes.
Other improvements to the fuel injection for this new engine were the use of composites and new-style seals for the mating surface. Also, a Throttle Actuator Control system (TAC) was implemented on the '97 and later Corvettes. This new system used motor control of the throttle body to control idle and operation of the engine for a complete drive-by-wire system without any throttle cables. This new engine with its new fuel-injection system put the base Corvette's engine at 345 hp--once again, getting close to that 1hp/ci milestone, only this time with much better driveability and fuel mileage. All because of the inclusion of electronics in fuel injection to help control fuel.
The MAF sensors changed as fuel-injection systems became more intricate. The '85-'89 Corvettes used a Bosch MAF (bottom), while '94 and later Corvette engines use an AC/Delphi MAF (upper left). Both MAF sensors operate on the same theory of a heated wire used to measure the amount of air entering the engine. The Bosch MAF operated on an analog signal of varying voltage, and the AC/Delphi MAF used a digital signal to the PCM. The Bosch unit emits a voltage signal, typically .04-5 volts; while the AC/Delphi unit emits a signal in the 32-150-mHz range which is converted from the voltage variation in the hot-wire circuit. The Bosch MAF also uses a Burn-Off cycle each time the engine is shut off to clean the wires within the MAF. The newer AC/Delphi unit does not use a burn-off cycle, so it's even more imperative to keep a clean air filter in your Corvette.
In '01, GM increased the size of the MAF from 74mm to 85mm, and included the Air Intake Temperature sensor in the MAF rather than further down the intake ducting. Also, the screen was removed from the '02 Z06 Corvettes (upper right).
GM returned to Bosch injectors for the LS1, although some changes were made over the years. The '97 and '98 Corvettes used 26-pound injectors. GM changed them to 24-pound injectors for the '99 and '00 production run, and bumped up to 28-pound injectors in '01.
From the very first efforts with fuel injection, the goal has always been the same: better fuel control, economy, and lower emissions. Achieve all three of these, and you will have reached the ultimate goal--higher performance. Where will it stop? Will we ever reach a performance plateau where further improvements will require a dramatic change? Possibly, but not yet. Fuel injection has evolved throughout the generations, sometimes revisiting ideas that had been shelved for a "better way to do it."
Currently, the buzz at GM is the idea of "dropping cylinders" by shutting off fuel to cylinders when the car could actually be powered by a six-, or even a four-cylinder engine. Owners of older Cadillacs may remember the 4100 series of engines that attempted to use this Displacement on Demand theory of 4-6-8 cylinder engines. Not widely received then, it was possibly too far ahead of its time. The processors in today's Corvette are so far advanced from what was available then, and the mechanical technology has advanced so much from those early pioneering days that, if the timing is right and the design is solid and reliable, this advanced technology may one day become just like all the others: a milestone in the ever-advancing evolution of EFI.